Skin is an interface between a living body and the external environment, and has a skin barrier function that prevents water loss from the body, and prevents penetration of biologically harmful substances, such as microorganisms, allergens and the like from the external environment. Keratin intercellular lipids, mainly ceramide, sebum and the like in the stratum corneum act as these skin barrier function. The stratum corneum must have a water content of 10 to 20% in order to achieve a normal function and maintain a healthy condition. Therefore, softness and the elasticity of skin are maintained by water retained in the stratum corneum due to the skin bather function. When the water content decreases in the stratum corneum, skin loses its softness and becomes hard, thereby cracks and the like may occur. So-called chapped skin of which patterns are invisible or unclear shows a significant decrease in water content in the stratum corneum. Rough skin not only has poor cosmetic appearance, but also is a preliminary stage to cause skin disease, and rough skin has a pathological significance. Further, by improving rough skin conditions, the surface of dry and flaky skin can be smoothed, thereby leading to improvement of fine wrinkles. It has been known that when the skin barrier function of the stratum corneum has deteriorated, water loss from skin is significant in comparison with healthy condition, that is, increased transepidermal water loss (TEWL) is observed. The TEWL is closely related with the barrier function and the moisturizing function of the stratum corneum, and is used as an index of the skin barrier function. Therefore, skin can be maintained in a healthy condition i.e., beautiful condition by increasing the water content in skin, or reducing the TEWL, or suppressing an increase in TEWL.
In recent years, studies on the mechanism of skin have been carried out, and it has been confirmed that macroscopic causes of dry feeling of the skin and rough skin are complexly involved in the effects of sunlight (ultraviolet rays), drying, oxidization, and the like in addition to effects due to decrement of the metabolism with aging. It has been found that these effects caused by such factors significantly decrease the amount of collagen fibers that are the main matrix component of the dermis. When a mechanism to keep tension and elasticity of skin that is maintained by the collagen fibers is destroyed by the effects of ultraviolet rays and the like, wrinkles or slacks of the skin increase. Since collagen molecules can retain water, and thus maintain skin in a moisturized condition, skin becomes dry and rough when collagen is destroyed due to external factors. Therefore, wrinkles and slacks of skin can be prevented by promoting biosynthesis of collagen that is one of the main components of the dermis layer, and skin can be maintained in a healthy condition, that is, beautiful skin condition. In recent years, deterioration in the skin condition of animals, particularly pets, due to allergy and the like has become a major issue. The skin condition of animals can be improved by moisturizing and caring the skin, healthy skin condition can be maintained.
A milk protein hydrolysate has been used for various commercial products in order to prevent food allergy caused by cow milk or dairy products. In particular, it has been considered that a whey protein of cow milk functions as an allergen, differing from a protein in mother's milk. Therefore, it has been known that a whey protein has been hydrolyzed using a protease in order to prevent this problem (see Patent Documents 1 to 3). It has been also known that a whey protein hydrolysate suppresses the progress of atopic dermatitis (see Non-patent Document 1), and improves curing wound after Cesarean section (see Non-patent Document 2).